Technical Field
A steerable beam antenna system using a phased-array of planar elements operating on several dissimilar frequencies or wavelengths.
Description of the Related Art
In a phased-array module with transmit and receive capabilities, it is desirable to have transmit beam aligned with receiver beam precisely. When the array antenna size is bigger, the beamwidth of the antenna beam is smaller and the required precision of alignment increases.
In a typical user terminal designed for mobility, the phased array antenna scans its field of view to find the incoming signal from the transmitter of a remote terminal or hub. When the receive antenna beam points to the correct direction, the incoming signal is received with high signal strength and demodulated. From the demodulated and decoded signal, the receiver acquires the proper status of the system operation and obtains some time window for its transmission. If the transmit antenna beam is aligned with the receive antenna beam, the signal transmission by the user terminal at the allowable time window of transmission can reach the remote terminal at proper strength (i.e., transmit signal toward the remote terminal enhanced with the high antenna gain) to allow the receiver of the remote terminal to process immediately.
If the transmit beam is poorly aligned with the receive beam in the phased-array antenna of the user terminal, a transmit beam training operation is performed in which the transmitter scans its signal across the region of the remote terminal to allow the remote terminal to acquire the signal at a local maximum. The remote terminal needs to feedback the status once it acquires the signal. Obviously, this operation is significantly more complex than the case in which a transmit beam is aligned with the receive beam.
When the phased-array antenna is being calibrated (the operation of aligning the transmit beam to the receive beam), the transmit AWV (antenna weight vector) is changed until the transmit beam precisely points to the same direction as the receive beam. This is usually performed within an anechroic chamber with a test antenna (which contains TX and RX) and the array antenna to be calibrated positioned within opposite sides of the chamber. The test antenna first transmits a signal to allow the phased-array antenna receive beam to adjust until peak power is received (meaning the receiver beam of the phased-array antenna is pointing at the test antenna direction). The phased-array antenna then transmits using different antenna beams (AWVs) until the test antenna received power is peaking. Note that in theory the AWV can be calculated mathematically based on the required phase shift values of each antenna element for a beam direction to compensate for different signal delays at antenna elements. However, in practice, due to hardware implementation imperfection, coupling in signal path for each antenna element within hardware, inaccuracies of implementations, physical misalignment, the mathematically generated AWV does not necessarily provide accurate alignment between transmit beam and receive beam.
There are a large number of AWVs (beams) in a large phased-array antenna. Phased-array antenna with n antenna elements consisting of n phase shifters. If the phase shifter has 2^k steps (a k-bit phase shifter), the number of possible AWVs would be 2^(k*n). A brute-force calibration going through 2^(k*n) AWVs can take extremely long time. Hence there is a need to uses a novel procedure to simplify the number of calibration states.
In principle, only a subset of receive antenna beam are needed. For example, if the beamwidth of an antenna of interest is 2 degree, the subset of antenna beams and its corresponding AWVs which are separated by 1 degree in pointing angle would be sufficient. The subset would cover the FoV with 1 degree beam step. This subset with 1 degree granularity is sufficient in practical operation. Small granularity can require a larger set of beams. The subset of AWVs is called codebook and the receiver beam points to each different direction by using an AWV within the codebook. The calibration of transmit beam is performed over each of the receive beam within the codebook.
A conventional phased-array antenna enables a highly directive antenna beam to be steered toward a single certain direction. The direction of an antenna beam may be controlled by setting the phase shifts of each of the antenna elements in the array. However, to enable higher mobility, the phase shifts must be updated more quickly than conventionally practiced. In addition, cost and space considerations eliminate the obvious deployment of parallel data buses. For sensitive RF circuits and interconnection in an phased-array antenna, it is also necessary to simplify and confine the amount of digital interconnection. Thus it can be appreciated that what is needed is a more efficient way of dissemination of the phase shift control information to a substantial number of phase shifters for an antenna array with a high number of antenna elements and possibly more than one simultaneous target.
Steerable single frequency phased-array antennas are known. Low Temperature Co-fired Ceramic (LTCC) devices are known. LTCC technology is especially beneficial for RF and high-frequency applications. In RF and wireless applications, LTCC technology is also used to produce multilayer hybrid integrated circuits, which can include resistors, inductors, capacitors, and active components in the same package. There are a number of similar low loss RF and high frequency substrates such as Rogers, Teflon, and Megtron 6, which are suitable for multilayer construction.
As is known, a planar antenna using layer substrate or LTCC (low temperature co-fired ceramic) or similar substrate material can be constructed using printed circuit board techniques.
As is known, a planar phased-array antenna consists of a number of antenna elements, deployed on a planar surface. Incoming planar waveforms arrive at different antenna elements of a receive phased-array antenna at different delays. These delays are conventionally compensated with phase shifts before the signals are combined. Conversely, a transmit array consists of a number of antenna elements on a planar surface, and the signals for these elements are phased shifted before they are transmitted to compensate for signal delay toward a certain direction.
      F    ⁡          (                        cos          ⁢                                          ⁢                      α            xs                          ,                  cos          ⁢                                          ⁢                      α            ys                              )        =            ∑              m        =        0                    M        -        1              ⁢                  ∑                  n          =          0                          N          -          1                    ⁢                                              A            mn                                    ⁢                  e                      j            ⁡                          [                                                m                  ⁢                                                            2                      ⁢                      π                                        λ                                    ⁢                                      dx                    ⁡                                          (                                                                        cos                          ⁢                                                                                                          ⁢                                                      α                            x                                                                          -                                                  cos                          ⁢                                                                                                          ⁢                                                      α                            xs                                                                                              )                                                                      +                                  n                  ⁢                                                            2                      ⁢                      π                                        λ                                    ⁢                                      dy                    ⁡                                          (                                                                        cos                          ⁢                                                                                                          ⁢                                                      α                            y                                                                          -                                                  cos                          ⁢                                                                                                          ⁢                                                      α                            ys                                                                                              )                                                                                  ]                                          
It is desirable to have a smooth element pattern which covers the array field of view (FoV).
For a planar phased-array antenna with antenna elements deployed with regular spacing in a grid, the spacing between adjacent elements must be less than a certain value, determined by its scanning angle, to prevent grating lobe.
Furthermore, the dimension of the antennas on a substrate may be optimized by the thickness of the substrate which would be desirably proportional to the wavelength or the inverse of the operating frequency.
Suppose a first antenna is designed to operate at a certain frequency. In order to preserve the same antenna properties (matching, bandwidth, gain, . . . ) at a second antenna for a second frequency, all relative dimensions of the second antenna design must be approximately inversely proportional to its second frequency.
Based on the above discussion, if a planar antenna is designed on a substrate, the thickness of the substrate should be approximately proportional to the inverse of operating frequency.
However, to generate a smooth antenna pattern with a wide beamwidth, it is necessary to have large enough ground plane—typically, ground plane size >λ×λ
Note that it is difficult, especially for antenna 2, to have sufficient size ground plane due to limited available aperture.
It is also difficult to obtain good isolation since the two antenna elements are separated by sub-wavelength distance.
What is needed is a method to reduce the number of calibration states for a large number of AWVs (beams) in a large phased-array antenna. A phased-array antenna with n antenna elements consists of n phase shifters. If the phase shifter has 2^k steps (a k-bit phase shifter), the number of possible AWVs would be 2^(k*n). A brute-force calibration going through 2^(k*n) AWVs can take extremely long time.